1.Field of the Invention
The present invention relates to a reflection diffraction grating for use in a pulse laser chain and having an improved laser flow resistance compared with a conventional metallic grating in the femtosecond regime.
2. Description of the Related Art
Today, in the fields of plasma physics or nuclear fusion, ultra-short pulse lasers (pulse duration shorter than 500 fs) with higher and higher energies are used in order to reach peak powers approaching the PetaWatt (PW), or even more.
However, the maximum reachable power is limited by the flow resistance of the optical components. The laser flow resistance of an optical component depends in particular on the surface energy density and on the pulse duration.
The problems of damage threshold of the optical components in high-energy pulse lasers have been partially solved by the technique of chirped pulse amplification (CPA). The CPA principle is to submit the light pulse to a time-spreading process, which reduces the peak power, to amplify it and, at the end of the laser chain, to time recompress it to obtain the desired short pulse. Thus, the light power during the amplification may be reduced by several orders of magnitude. However, it remains a risk to destroy an optical component in the stage that performs the pulse compression, based in particular on the use of diffraction gratings. Some of those components are indeed exposed to the energy-amplified and time-compressed pulse, having thus the highest peak power. The diffraction gratings of the compressors are thus limiting components in terms of flow resistance.
The diffraction gratings for pulse compression were first conventional metallic gratings. For pulse compression in the infrared domain (at 800 nm, 1053 nm or 1550 nm), there is no use of aluminium gratings because their diffraction efficiency, generally lower than 90%, is not sufficient. Instead, gratings covered with a layer of gold are used. The gold-based gratings offer an excellent diffraction efficiency over a wide spectral bandwidth and require no protective layer because the gold is an inoxidizable material. However, the gold-based gratings suffer from a limited laser flow resistance in the femtosecond regime. Therefore, for the femtosecond domain, with pulses shorter than 500 fs, the damage threshold is of the order of 0.2-0.3 J/cm2 for the conventional gold-based gratings.
A first solution to permit increasing the laser power is to increase the size of the beams and of the optical components so as to reduce the surface illumination. But increasing the size of optics, in particular for the diffraction gratings, rapidly comes up against technical limitations of production as well as against a significant increase of the fabrication cost. There is thus a great interest in increasing the flow resistance of the diffraction gratings.
Another solution to further increase the diffraction efficiency and the flow resistance has been to fabricate diffraction gratings on dielectric mirrors (MLD: multi-layer dielectric). An MLD grating generally comprises an alternating stack of a great number of layers made of two fully-transparent dielectric materials having different optical indices and alternating in the thickness direction, and a grating formed in the last thin layer, at the surface of the multi-layer stack. Such MLD gratings are described in detail in many articles, for example: “Design of high-efficiency dielectric reflection grating” by Shore et al., JOSA A, Vol. 14, Issue 5, pp. 1124-1136, “High-Efficiency Dielectric Reflection Gratings: Design, Fabrication, and Analysis” by Hehl et al., Applied Optics, Vol. 38, Issue 30, pp. 6257-6271, “Design of diffraction gratings for multipetawatt laser compressors” by Bonod et al., Proc. SPIE, Vol. 5962, 59622M (2005).
These publications recommend to fabricate diffraction gratings from fully dielectric, transparent and without absorption materials, comprising a high number of bilayers, so as to obtain MLD gratings with a flow resistance two to three times better than that of the gratings having only one layer of gold. In theory, the MLD gratings have also a diffraction efficiency higher than that of the gold-based gratings. The MLD gratings thus progressively replace the gold-based metallic gratings in the very high intensity pulse compressors.
However, the MLD gratings are more complicated to fabricate than the metallic gratings and are thus more expensive. Moreover, the MLD gratings have a too limited spectral bandwidth (a few tens of nm) to be used in ultra-short pulse (<50 fs) laser chains. Indeed, the duration of the laser pulse is Fourier transform-linked to the spectral bandwidth of the laser, which means that the product of the pulse duration with the spectral width of the light radiation is a constant. By way of information, at the central wavelength of 800 nm, which is commonly used today, this product is equal to about 1000 fs.nm, which means that to obtain a pulse with a time width shorter than 10 fs, a bandwidth wider than 100 nm is required, i.e. a very high efficiency bandwidth (>90%) over a wavelength domain surrounding the central wavelength of interest. A MLD diffraction grating cannot have such bandwidth performance. The MLD gratings have a bandwidth typically lower than 50 nm at the central wavelength of 1053 nm.
The flow resistance of the optical elements (materials, mirrors, diffraction gratings) exposed to laser pulses is still a vast domain of investigation, wherein all the phenomena are not yet explained. The damages caused to the materials due to the laser flow in the nanosecond to picosecond pulse regimes are rather well known today. In the femtosecond domain, new phenomena occur and the damage mode is different.
In the picosecond and nanosecond regimes, the main phenomena are of thermal nature and are linked to the absorption, in particular as regard the metallic gratings. Whatever the material is, the damage threshold follows a square root law of the pulse duration. The following articles describe a number of measures and models of laser damage on mirrors and diffraction gratings: “Optical ablation by high-power short-pulse lasers” by Stuart et al., JOSA B, Vol. 13, Issue 2, pp. 459-468, “Short-pulse laser damage in transparent materials as a function of pulse duration” by Tien et al., Physical Review Letters, Volume 82, Issue 19, May 10, 1999, pp.3883-3886.
For femtosecond pulse durations, this law is not followed, the physical phenomena at the local scale of a grating line then appear to be linked to the square of the electric field of the electromagnetic lightwave in the materials. It is thus demonstrated by the following articles: “Multilayer dielectric gratings for petawatt-class laser systems” by Britten et al. Proceedings of the SPIE, Volume 5273, pp. 1-7 (2004), “Effect of electric field on laser induced damage threshold of multilayer dielectric gratings” by Neauport et al., Optics Express, Vol. 15, Issue 19, pp. 12508-12522, that the damages in diffraction gratings in the femtosecond regime (pulse duration shorter than 500 fs) is strongly linked to the square value of the electric field in the material forming the profile of the diffraction grating lines.
Indeed, for very efficient diffraction gratings (i.e. whose diffracted energy is almost fully concentrated in the useful diffraction order (the order −1 for this type of grating)), stationary waves are formed due to the interference of the incident field with the diffracted field, and the electric field may have an amplitude of twice that of the incident field near or inside the material, which is referred to as “reinforcement of the electric field”.
A conventional metallic diffraction grating operates in TM polarization with a metallic treatment, usually gold. The electric field at the metal and the metal-vacuum interface presents areas of high field-reinforcement at some points of the line profile that constitute the weakening areas regarding the flow resistance.
The laser flow resistance depends of course also on the quality of fabrication: purity of the materials used, density of the materials, absence of impurities or defects (cracks, inclusions, bubbles, roughness).
The type of material used has logically also a great influence on the flow resistance, as well explained in the following article about different transparent materials: “Scaling laws of femtosecond laser pulse induced breakdown in oxide films” by Mero et al., Phys. Rev. B 71, 115109 (2005).